Polymers of carbon monoxide and olefins generally referred to as polyketones are well known in the art. The class of linear alternating polymers of carbon monoxide and at least one ethylenically unsaturated hydrocarbon are of particular interest among polyketone polymers. This class of polymers is disclosed in numerous U.S. patents assigned to Shell Oil Company, exemplified by U.S. Pat. Nos. 4,880,865 and 4,818,811 which are incorporated herein by reference.
Polyketone polymers display a well balanced set of mechanical properties which make them particularly useful as engineering thermoplastics. The excellent properties of polyketones could be further exploited by further improving the materials so that they exhibit improved tribological properties. Such parts and items made from such materials would be able to resist wear and bear greater frictional loads when in rolling or sliding contact. This would be particularly desirable over extended durations. Such properties are generally attained through the addition of new or additional additives. It would be beneficial if they could be attained without the addition of additives to avoid other changes in mechanical properties due to additive loading of the polymer matrix. It would also be beneficial if other attributes such as thermal performance, dimensional stability, and tensile strength could be improved without further loading the polymer with additives.
It is known that polymers containing ketone groups degrade according to Norrish type I and/or Norrish type II scission reactions upon exposure to UV radiation. This occurs either in the presence or absence of oxygen. We have found that alternating polyketones irradiated in the presence of oxygen undergo significant molecular weight loss resulting in a polymer having undesirable properties such as brittle behavior and loss of mechanical strength. When the irradiation occurs in the absence of oxygen, similar scission reaction occur initially, however, a crosslinked polymer is formed which also suffers from embrittlement and loss of mechanical strength. The latter crosslinked materials have significant gel fractions (greater than about 10%).
It is also known that linear alternating polyketones can undergo crosslinking in the melt through ionic reactions such as aldol condensations and the like if the polymer has a significant heat history. These crosslinked materials suffer from a loss in ductility and toughness and often possess discrete gel particles in the polymer matrix. Polyketones which undergo this type and degree of crosslinking (in the melt) also exhibit reduced crystallinity and/or crystalline melting point. Furthermore, such materials are destabilized toward oxidative degradation. In short, polyketones which are crosslinked in the melt are not particularly desirable. Aside from chemical modification/reaction, substantially crosslinked alternating aliphatic polyketones which do not exhibit such a loss in mechanical properties have not been heretofore produced.
The exposure of polymeric materials to electromagnetic radiation can also affect both the physical and chemical properties of the material. One way in which polymers can be affected is through crosslinking. Crosslinking is the attachment of at least two chains of polymer molecules. It can be accomplished through the use of a bridge which bonds to at least one atom along each polymer chain. Alternatively, crosslinking can occur through branching along the polymer backbone. When crosslinking is extensive a network may be formed which is insoluble in typical solvents used in the uncrosslinked polymer.
Curing, as the term is used generally, refers to the treatment of a starting product to produce a finished product which is more useful for a particular purpose. Polyethylene, for example, can be cured to form various useful crosslinked versions of the polymer through .gamma.-irradiation. With respect to polyketones, it has been hoped that the following useful properties could be improved through a curing process: wear properties, thermal properties (eg. heat deflection temperature and melt viscosity), barrier properties, chemical resistance properties, creep and fatigue properties. Retention of ductility, tensile strength, and impact strength are also important during cure.
In some applications, exposure to electromagnetic radiation is affected for a purpose other than crosslinking or curing. For example, sterilization against biological contamination is often conducted through the bombardment of vessels with .gamma.-radiation. Property improvement is not necessarily an objective of such decontamination but it is important that the materials not significantly lose mechanical properties such as ductility, tensile, and impact strength.
U.S. Pat. No. 3,812,025 to Guillet proposes radiation crosslinking of polyolefins having a small mole fraction of carbon monoxide as monomer (generally between about 1 and 10 mole %). While Guillet also presents the possibility of crosslinking polyolefins with up to 50 mole % ketone groups, such crosslinking was expected to be accompanied by Norrish I/II chain scission reactions. Guillet's article in the Journal of Polymer Science describing this same work presented in this patent explains why this is so. There he states that the photolysis attributable to Norrish type I and Norrish type II reactions increases with decreasing aliphatic/ketone lengths (greater mole fraction of carbon monoxide). J. A. Slivinskas and J. E. Guillet, .gamma.-Radiolysis of Ketone Polymers, Journal of Polymer Science, Vol. II, 3043-3056 (1973). This view would be in accordance with the understanding of the degradative processes set forth above. Thus, Guillet's work suggests that irradiated alternating polyketones should undergo simultaneous scission and crosslinking resulting in a crosslinked material which would not retain ductility, impact, and tensile strength. Any crosslinked material so formed would be expected to be comprised of relatively low molecular weight fragments between crosslinks. Thus, it can be seen why Guillet restricted radiation crosslinking to polymers having a large aliphatic chain length to ketone portion of the backbone. The examples and claims of the patent are drawn to materials having a maximum of about 5 mole % carbon monoxide.
Thus, the prior art would suggest that any improved effect that could be hoped to be attained by radiation curing would have been offset by the more likely outcome that polyketone polymer would undergo degradation. It has now been found that the properties of polyketone polymers can be enhanced through high energy curing without such degradation and that the materials produced from this process are unique.